Since the inception of Ion Chromatography (U.S. Pat. No. 3,920,397), the basic technology for the anion-exchanger used in the analytical column for anion determination has not changed. Microparticles of anion-exchanger (0.1 to 5.mu.) are agglomerated with macroparticles (5 to 100.mu.) of surface sulfonated or fully sulfonated styrene divinylbenzene copolymer to produce a low capacity "pellicular type" anion-exchanger (U.S. Pat. No. 4,101,460).
Improvements within this basic technology came with the use of monodisperse anion-exchange latex rather than the previously used ground anion-exchange resin (as described in the '460 patent), and by performing the agglomeration step in a polyvalent salt solution (U.S. Pat. No. 4,119,580). The use of monodisperse anion-exchange latex eliminated the problem of refining ground ion-exchange resin to obtain the desired size range, while agglomerating in a polyvalent salt solution resulted in a reproducible and dense deposition of microparticles due to the resulting suppression of the ionic repulsion forces between the microparticles.
State of the art anion exchangers of this type show a performance level in which baseline separation of fluoride, chloride, nitrite, phosphate, bromide, nitrate, and sulfate anions is achieved in about 20 minutes using a one-half meter column, e.g., as illustrated by the chromatogram of FIG. 3 of the '580 patent.
The calculated theoretical plate count (N) for the bromide ion in this separation is N=.about.650, and for the sulfate ion, N=.about.500, using Equation 1, below, and measuring retention time from the leading edge of the little retained fluoride peak to correct for column void volume effects: EQU N=T.sup.2 /W.times.16=theoretical plate count Equation 1
where
T=retention time, in minutes PA0 W=triangulated peak width at baseline, in minutes.
Plate counts of about 650 for a 1/2 meter column are considered quite low by current liquid chromatographic standards, but about as expected for a pellicular packing (Bulletin No. 123, Whatman Inc., Liquid Chromatographic Product Guide, page 3 (1977)). Today, most liquid chromatography is practiced using the "microparticulate" type of packing, based on 5-10.mu. porous silica particles, because of their superior efficiencies (sharper peaks). This results in faster analyses, better detection limits, and better separation of interferences. Plate counts of 2,000-10,000 per 250 mm are common for microparticulate packed columns. All other factors the same, a column with four times the plate count will result in a twofold improvement in resolution with peaks about twice as tall. Thus, performance can be linearly compared by comparing the square root of N.
Based strictly on efficiency criteria, a silica based column, such as Whatman's Particil-10-25 SAX (strong anion exchanger), would thus appear to be strongly preferred for use in anion analysis versus the current pellicular form of column. However, prior attempts to use silica based columns for this application frequently prove unsuccessful, due in part to serious dissolution problems.